Dell XPS15 + TD15 is the biggest fail of 2015 and 2016.
This is unbelievable example of software team incompetency.
If XPS15 works in common stable after 6+ months after sales started.
While TB15 not only lags, but every new software update adds new lags and bugs, even not always fixing previous. This is unbelievable worst software processes.
And this is one of the best PC laptop and one of the best Thunderbolt dock.
The problem is not new:
I have Dell XPS15 9550 + TB15.
Sure this doesn't work with a closed lid with any configuration.
But it doesn't work even with opened lid. I used this manual to configure and update drivers/firmware.
It was working for me in June, and first part of August (sure with lags, but I was able to work with this).
But then, suddenly %), without any updates, lags frequency increased. And my external monitor became turning off every 1-10 minutes. Sound and mouse too.
Without any updates, like a magic.
After updating from scratch of all drivers and firmware (the same manual), nothing changed.
Actually I think monitor works with more lags.
As a customer who used V131 laptop, Dell monitor for many years, it looks like that were last products I purchased..